GCC/Clang: Turn -Wdangling-reference into error, if available

you usually get that when calling a method that returns a reference on
a temporary and assigning that to a reference variable.
Often nasty do debug...

Example:
idVec3 v1 = ...;
idVec3 v2 = ...;
const idVec2& cross2d = v1.Cross(v2).ToVec2();

idVec3::Cross(const idVec3 &a) returns idVec3, i.e. a temporary, and
idVec3::ToVec2() returns a reference to the idVec2 part of an idVec3.
That temporary is gone after that line and cross2d points to invalid
memory..

(I found this change in another branch, don't remember what triggered
 me creating it, might even have been a bug in another project, but
 either way it's great that compilers can now warn about this and
 if they do that should be treated as an error and be resolved.)

Author: DanielGibson

neo/CMakeLists.txt

@@ -417,6 +417,11 @@ if(D3_COMPILER_IS_GCC_OR_CLANG)
 	add_compile_options(-Wno-sign-compare)
 	add_compile_options(-Wno-switch)
 
+	CHECK_CXX_COMPILER_FLAG("-Wdangling-reference" cxx_has_Wdangling_reference)
+	if(cxx_has_Wdangling_reference)
+		set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=dangling-reference")
+	endif()
+
 	CHECK_CXX_COMPILER_FLAG("-Woverloaded-virtual" cxx_has_Woverload_virtual)
 	if(cxx_has_Woverload_virtual)
 		set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Woverloaded-virtual")